GB2263844A - Communication systems - Google Patents

Abstract

A communication system for a building equipped with coaxial cabling for transmission of television signals, as in an MATV system, wherein the same coaxial cabling (18) is used in a telephone system for calling up and subsequent speech by frequency modulation of carriers of different frequencies for forward and return paths, between a door entry station (10) and/or a concierge station and any one of a number of apartments in the building. <IMAGE>

Description

Title: Communication Systems Field of the Invention This invention relates to communication systems.

Backqround to the Invention Buildings such as blocks of flats are commonly equipped with wiring, normally coaxial cables, for feeding television signals from an antenna to a plurality of TV socket outlets, one in each apartment of the block. This use of a common antenna to feed TV signals to a number of TV socket outlets is commonly called a MATV (master antenna television) system. Where the antenna receives TV signals from a satellite the system is called a SMATV (satellite master antenna television) system. The invention stems from the realisation that it is possible to use this wiring to transmit other data related to the functioning, maintenance or condition of the building.

Disclosure of the Invention According to one aspect of the invetltior a comrr,unication system for a building comprises wiring for feeding television signals to a plurality of occupants of the building, and a telephone system comprising a caller station and a plurality of user stations, whereby a caller at the caller station can establish telephone communication with an occupant at a selected user station, said wiring also serving to transmit the telephone signals between the caller station and the selected user station.

The caller station may be a door entry station in a door entry system which controls the locking mechanism of a door or gate giving access to the building or grounds surrounding the building. In this case, the door entry station includes a door release mechanism which is operated to release the door on receipt of a door release signal from a selected user station. The user stations are then located in respective apartments of the building which may be equipped with a further caller station for use by a concierge or the like.

The wiring may be part of a MATV or SMATV system and the caller station is preferably connected into said wiring by means of a coupler, such as a diplexer.

The caller station and selected user station preferably set up a two-way speech channel by frequency modulation of a carrier, and in the preferred embodiment the transmission of speech from the caller station to the selected user station is at a first frequency (eg 47.3 megahertz) and the transmission of speech from the selected user station to the caller station is at a second frequency (eg 21.4 megahertz).

The caller station preferably comprises addressing means (eg a keypad) for addressing a selected user station and for transmitting an address signal to all user stations, each of the latter having address recognition means for recognising the particular address assigned to the user station. On recognition of an address signal, a selected user station preferably transmits to the caller station a reply signal, representative of the address of the selected user station, the caller station having means to compare or match the address signal and the reply signal and to establish the speech channel between the caller station and the selected user station on a match being achieved. The speech channel thereby established is preferably secure in that neither can the channel be interrupted, nor can the speech be overheard, by another occupant.

Each user station preferably has a splitter which splits the TV signals transmitted on the wiring from the telephone signals which are also transmitted on the wiring. Each user station preferably has a socket for a standard plug-in telephone, one key of which is allocated to generate the door release signal.

According to the invention, in another aspect, a communication system for a building comprises wiring for feeding television signals to a plurality of occupants in the building and a data transfer system comprising a plurality of user stations connected to a caller station by said wiring, each user station having respective recognition means operable to open a channel for transfer of data from the caller station to the user station in response to an address signal which contains an address assigned to that user station, the arrangement being such that the caller station selects which user station is to receive the data to be sent by sending to all user stations an address signal which operates the recognition means of the selected user station.

Preferably the communication system includes calling means on each user station for transmitting a distinctive call signal to the caller station which includes identification means for identifying the user station from which the call signal was sent.

Such a call signal may be transmitted in response to, for example, the triggering of a smoke alarm or an intruder alarm, and the identity of the user station transmitting the signal may be displayed on a display connected to the caller station. Such alarms preferably provide a continuous monitoring facility so that a call signal is transmitted upon triggering of the alarm, regardless of whether any address signals have been sent from the caller station.

Description of Embodiment A description of a preferred embodiment of the invention follows, making reference to the accompanying drawings, in which: Figure 1 is a block diagram illustrating the overall system; Figure 2 is a block circuit diagram relating to an apartment user station; Figure 3 is a block circuit diagram relating to a door entry caller station; Figure 4 is a block circuit diagram relating to a concierge caller station; Figure 5 illustrates the system data structure; and Figure 6 illustrates the call set-up protocol of the system.

General The illustrated system comprises a door entry system in which the conventional twisted pair connections between individual apartments and a door entry unit are replaced by the use of radio frequency carriers over the existing MATV/SMATV coaxial network, installed in blocks of apartments for the provision of television entertainment services.

A simple but effective control protocol is incorporated, which gives ready communication between called apartments and the main door unit, and may also support more sophisticated features such as remote automatic detection of security devices such as smoke alarms and intruder detectors, and make possible enhancements to services such as remote meter reading and general building control and monitoring.

A preferred optional feature of the system is a concierge panel which allows at-a-glance identification of alarm location and type, as well as conversation between the concierge and individual apartments over the coaxial network.

The illustrated system has the following features, as will be apparent from the following description: - Individual signalling from entrance to apartments - Concierge or caretaker panel with door entry control - Duplex, secure speech channel between apartment and entrance - Door release remote signalling - Privacy feature, disabling the called sounder when required.

However, additional features may be incorporated, including some or all of the following: - Smoke alarm input - Intruder alarm using (for example) movement or IR detectors - Main door video channel - "Panic button" facility to call concierge direct The illustrated system is intuitively simple to use and is reliable in operation. In particular, to the casual user it appears identical to a conventional door entry system.

Essential to the system is the use of a TV existing coaxial distribution network for the carriage of all signalling; no additional wiring needs to be routed to individual apartments.

The Overall System The system implementation is shown in Figure 1. This approach is applicable to a number of distribution architectures, but is especially suited to an architecture using a single trunk drop with subsidiary tap amplifiers feeding groups of drops.

The amplifiers (Teleste ADA408) employed offer ample gain over a band of 47MHz to 860MHz; a backward channel of band width 5MHz to 30MHz is provided by an ADR30 module, but without gain. The units are line powered, but power is not supplied to individul drops.

Reference to Figure 1 shows a dedicated drop to the door entrance unit, generally referenced 10, which is combined with the main system signals at the headend 12 using a bidirectional diplexed coupler 14. Communication signals from the door entrance panel are carried throughout the system, via the subsidiary tap amplifiers 16, to all the individual drops. Each drop is associated with an apartment unit, which is capable of being individually addressed by a signalling protocol later described. The backward channel from apartment to door entrance unit is also carried via the ADR30 units (i.e. the splitters and taps used are bidirectional) onto the main trunk 18, and so back to the door entrance unit 10. By careful choice of the system signal frequencies and levels, full duplex communication can be achieved using existing SMATV cabling, with the minimum of modifications.

The Apartment Unit User Station This unit, shown in Figure 2, comprises a skirting mounted enclosure, approximately 15 cm by 4.5 cm, containing the communication circuitry 20 which interfaces with the coaxial system, including the protocol handling subsystem which enables each apartment unit to be called uniquely over the network and to set up a secure, private conversation, and the telephony circuits 22 which interface to a standard BT specification phone 24 (with DTMF signalling). An additional input facility 26 is provided for the connection of automatic devices such as smoke detectors, intruder devices, etc.

A standard phone 24 has a number of key advantages; no tooling is required to produce an attractive unit, it is inexpensive, has an inbuilt signalling capacity via the numeric keypad, and is easily replaced in the event of a fault. Additionally, the apartment dweller may choose to use an alternative unit such as a cordless phone or even an answering machine, wihout compromising the system integrity. Connection of the phone 24 to the apartment unit is by a standard BT jack 28.

Where possible, the skirting unit will be capable of being powered over the coaxial network; an inbuilt battery capacity 30 will reduce standing demand to a very low trickle current. This facility also provides some protection against interruptions to the mains supply.

Alternatively, for systems where powering over the coax is not practicable, a d.c. input jack is provided to allow a "plug top" mains transformer supply 32 to be used.

The communications circuitry housed in the unit will be clear from the labelled block diagram of Figure 2, when considered in the light of subsequent description herein.

The Door Entry Caller Station The door entry unit shown in Figure 3 is straightforward to use, performing identically to door entry systems currently in use. A push button matrix 34 sets up a call signal to individual apartments. A maximum of 240 apartments is allowed for in the electronics 36 of each door entry unit. When a call is answered, a loudspeaking telephone subsystem 38 allows a full duplex conversation with the called apartment; the call setup protocol prevents other apartments "listening in" on the conversation. The apartment dweller is able to use the telephone handset keypad in the apartment to admit entry to the caller or, by hanging up, terminate the call.

Special precautions are taken to prevent faulty, absent or off hook apartment units "blocking" the system.

No other indication is given to the caller as to whether the apartment is occupied or not, or indeed whether a particular apartment unit is connected to the system or not. This important security feature prevents possible searching of the block for empty or vacant apartments via the door entry system.

Normally, one push button is reserved for the caretaker or concierge.

The block circuit diagram of the door entry unit shown in Figure 3 will be clear from later description herein.

However, attention is drawn to the door entry control 40 and latch-operating solenoid 42.

The Concierqe Caller Station The concierge office or apartment can be equipped with a refined version of a standard apartment unit (Figure 2) whIch gives indication of calls from apartments, either set up by automatic sensor devices or by the apartment dwellers themselves. The additional features of the concierge unit are shown in Figure 4. Thus, the unit houses a duplicate 44 of the door entry panel, scaled down and without the loudspeaking phone, which enables the concierge to make calls to individual apartments.

Incoming automatic sensor or "call concierge" signals are displayed on display panel 46, identifying the number of the calling apartment. The most straightforward type of indication is an LED associated with each apartment pushbutton, glowing constantly or flashing according to the type of call. The concierge is then able to determine at a glance which apartment has triggered the panel, and what type of action may be required. An additional feature may be an audible alarm to draw attention to particular types of call. It is important to identify each call independently, as the system will need to handle multiple calls at the discretion of the concierge. If the concierge chooses to call a particular apartment in response to the call, he simply presses the corresponding button on the keypad 44: he then is able to talk to the apartment exactly as if he were an outside caller.

As the concierge unit uses the same network resource as the door entry unit, so the concierge is required to use the system lightly to prevent blocking access from external callers. However, even allowing for this restriction, the concierge call and monitoring service is a useful addition to building wide security, and cannot be achieved easily with conventional systems. Addition of features such as remote meter reading is readily possible.

Door Entry Video Facility Referring back to Figure 1, a video camera 48 may be provided to feed a camera signal back to the headend 12, where a conventional video to RF modulator can be used.

Provided that there is reasonable protection from interference, the attenuation of coaxial cable at video baseband frequencies will be negligible.

If available for the frequencies used in the door entry system, a diplexer may be used to couple the video signal in with the coaxial drop to the door entry unit.

Modern consumer grade cameras are attractive for this facility, not only because of cost, but also since modern CCD image sensors have become standard on consumer cameras, the sensitivity under low light conditions has become outstanding, which is a vital requirement for any video security application. Furthermore, the range of lenses now available enables the field of view to be readily set up on installation of the camera. Clearly, a secure mounting location, hidden if possible, is desirable.

In the system illustrated, no special treatment of the video signal is envisaged; a single VHF or UHF channel is dedicated to the door entry camera, enabling residents to select the door entry channel at any time. In R more refined system, it is possible to make access to the camera channel conditional on having first been called from the door entry panel.

Operatinq Protocol In reading the ensuing description, reference should be made to the block circuit diagrams of Figures 1 to 4, as well as to Figures 5 and 6.

The system operating protocol defines the sequence of events following the pressing of a call button at the main entrance. The protocol can be defined in three distinct phases: (a) Call setup, in which a particular apartment unit is addressed over the coaxial network triggering the called sounder and the system is prepared to carry a conversation; (b) Call progress, in which the full duplex speech channels are set up and maintained for conversation until the third phase; (c) Call termination, involving the admission (or refusal of entry) of the caller, and the relaxation of the system in readiness for another call.

The protocol design allows for all possible combinations of events, and offers a rigorous route to eventual system relaxation, avoiding possible lockups or "hung calls", which may block the system entirely.

Call Setup Call setup routing occurs as a result of one or more of a number of stimuli; for each case the protocol needs to set up a call to either an individual apartment or to a nominated "special" unit such as (for example) the concierge desk. In the following table some examples of stimuli and the corresponding call setup routes are given: Stimulus Setup Route - a caller at the door Entrance > ? apartment - apartment "help" call Apartment > ? concierge - smoke alarm Alarm unit concierge - intruder alarm Alarm unit concierge The above examples show the range of setup routes that are desirably accommodated in a simple system. Additional services such as remote meter reading require more complex setup routes.With a conventional twisted pair network, some form of physical exchange is required to control routing. In the coaxial system of the invention, call routing is achieved by assigning "addresses" to each unit on the network, which are used to identify the call destination, and the source of the stimulus when necessary (for example, during an alarm call).

All of the protocol sequences described below depend upon the generation, transmission and decoding of unique addresses, corresponding to individual apartment units, and the subsequent conditional transmission of data codes to set up secure communication links. Readily available integrated circuits incorporate the hardware and software to accommodate this, together with the associated error detection and masking routines.

These circuits make the following protocol achievable without resorting to any software development, with associated reductions in development costs and time scales. The circuits use a compacted digital transmission standard, especially tailored to remote sensing and control applications, known as trinary state signalling. In this format, three states are possible rather than the two of binary logic; this substantially reduces the amount of data that needs to be signalled around the system in order to control communication.

Figure 5 shows the structure of the data signal. The structure is a 9 bit trinary signal giving 39 (ie 19683) possible combinations. The first five bits constitute the address field A and the final four bits constitute the data field D.

In call setup protocol signalling, the unit addresses consist of a digital signalling sequence, carried throughout the network on a single pair of transmit/ receive frequencies. Only one unit on the network will respond to any particular address "call": all others will ignore it.

An example of a system transaction is shown in Figure 6 where the letters A and D refer to the address and data fields respectively. When triggered (in the case of the door entry unit, by pressing an apartment call button) the calling unit must first "listen in" on the relevant frequency to ensure that no other sequence is in progress.

If desired, a "System in use" indication can be provided.

If clear, the called unit address is transmitted repeatedly until either a timeout occurs, say after 20 seconds, or another button is pressed, or the called unit responds with its own address "echoed back" on the receive frequency, which occurs when the called unit handset is picked up. If timeout occurs (i.e. no answer) the system returns to the relaxed state until a further call sequence is initiated. If the handset is picked up, then the exchange of address signals opens up the speech channels, allowing a full duplex conversation to take place.

This simple protocol also enables a basic privacy feature, in that the duplex link is only established following a complete exchange of addresses, which is triggered by lifting the handset in the apartment. If the handset is left off the hook without a call being set up from the door entry unit, then no transmission will take place from the apartment unit even if called from the door entry unit in the normal way. Furthermore, no possibility of "listening in" to an apartment with an off hook handset is possible.

During call progress, full duplex conversation is enabled between the calling unit and the called unit. However, control resides with the called unit at all times.

For a door entry unit, conversation takes place via a "handsfree" loudspeaking telephone system, while the apartment units use the conventional telephone handset.

The speech channels themselves are carried over a pair of fm carriers, referenced 50 in Figure 6, these being the same carriers used to convey the address information over the system; the data signals are prevented from being heard by (a) muting circuits in each of the units, and (b) by choosing a data rate which is below the audio range of the telephony channels.

It is only possible to hold a conversation between calling and called units once the exchange of addresses has been carried out; this security feature prevents the possibility of "eavesdropping" over the system.

At the end of a conversation, if door entry is to be permitted, the telephone handset keypad can be used to signal to the door entry unit. As with the preceding conversation, this can only occur following the handshaking of addresses described above: this prevents the door being opened inadvertently.

For controlling door entry, the DTMF dialling tones provided with the handset are used; they are carried over the speech channels just as is the case with PSTNs, and decoded at the door entry unit to activate the door entry solenoid 42 (Figure 3). A fixed timeout of about 5 seconds is provided. The DTMF tones are only decoded on the return channel from the apartment unit; it is therefore impossible to gain entry illicitly by using, for example, a pocket DTMF autodialler.

Finally, when the called unit has the handset replaced, this action initiates the closedown sequence, in which the called unit address is retransmitted, but with a data field which forces the termination of the call. This special code ensures that the system is returned unambiguously to the relaxed state, irrespective of whether entry has been permitted or denied, ready to handle the next stimulus. As an additional protection against a hung call, i.e. failure to satisfactorily terminate, the door entry receive channel incorporates a carrier detector 52 and vox detector timeout 54 (Figure 3). In the event of the receive carrier being lost in mid call, or alternatively being received for a period of 20 seconds without any voice or data modulation, then the transmit signal is terminated and the entire system cleared down to the relaxed state.This last provision protects against the remote possibility of a faulty apartment unit "locking up" the system when called from the door entry panel, or the more likely case of a handset not being correctly replaced.

Figure 6 shows the transactional sequence involved in call setup, progress and termination, and will be clear without further description.

The call termination sequence makes use of a special buried code, which is always transmitted along with a unit address. The code is used not only by apartment units, but automatically by remote sensors such as smoke detectors, which are required to signal to a special address (usually, to the concierge unit) and then they must immediately clear down the system to be ready for the next stimulus; this prevents the system being rapidly saturated in the event of multiple sensor triggering events.

The data field can also be used to identify the location of a remote sensor, or in more advanced systems can be used for special signalling schemes such as (for example) a backward channel to the headend for use by the system operator/owner.

In order to incorporate automatic signalling, the apartment unit has a number of auxiliary inputs (say, four) at unit 26 (Figure 2), which can be triggered by a simple open/closed switch command from an external device (e.g. a smoke alarm). Each of the four inputs corresponds to one of the special buried codes, which are transmitted along with the apartment unit address in the normal way. At the concierge unit, this signal can be used to immediately determine both the location and type of the alarm. In order to minimise the occupancy of the transmit carrier, automatically generated calls are limited to 500 msec per call, repeated every 60 seconds while the corresponding input stimulus exists.

It is also provided for an apartment to call the concierge unit, and set up a call over the duplex speech links. As only a single carrier is available for the whole system, it is very desirable that the minimum amount of time is spent setting up and carrying out calls between concierge and any apartment. For this reason it is proposed that the automatic signalling routine previously discussed is used, that is, an auxiliary input is set up to be a "call concierge" button which signals the apartment address on the concierge unit, with the appropriate special code being indicated. It is then up to the concierge to return the call, using a duplicate receive/transmit unit to that in the door entry unit.It is important to note, however, that this scheme requires a special connection to be made to the main trunk of the network, otherwise the forward and return path gains will be incorrect for the receive and transmit carriers. A convenient spot may be at the trunk final termination point.

This simple approach enables conversations to be set up from either end, but always at the discretion of the concierge. This enables him to handle multiple calls in turn, and to call apartments whose automatic alarms have been activated to find out if anyone is there.

Radio Requirements As the door entry system uses frequencies in the 20 to 50 MHz range (permissible within the constraints of MPTl5l0, Appendix C), cable attenuations of between 10dB/lOOm and 6dB/lOOm can be expected.

A breakdown of the estimated link loss through the system of Figure 1 is as follows: Forward link loss: 40dB max Backward link loss: 83dB max The difference in the two link losses is, of course, due to the gain of the trunk amplifiers (40dB) in the forward path, and their loss (estimated at -3dB) in the return path.

Assuming similar designs of receiver in the door entry and apartment units, and specifying their sensitivity at -90dBm, then the apartment units must transmit at a level of -5dBm, and the door entry unit at only -50dBm. These figures include a lOdB margin to allow for additional unspecified losses or interfering sources.

These power levels are easily achievable with low cost devices and comparatively simple circuit topologies, using single chip-receivers such as TDA7000, TDA7010 (Philips) or MC3361 (Motorola) and discrete transistor VCO modulators and amplifiers for the transmitters. These circuits are used extensively in narrowband fm communication applications (such as cordless and cellular phones) and so are both inexpensive and well documented, and need no further explanation here.

In order to protect against possible patterning on optional vision services, the transmitters must be spectrally pure; this is particularly true in the case of the apartment unit where there is possibly zero attenuation between the transmitter output and the TV input. If the worst case TV signal level is about -85dBm (i.e. 0.5mV into 75 Ohms) and a protection ratio of 45dB is required, then spurious and harmonic outputs of the transmitter must be held below -130dBc throughout the TV bands to prevent possible patterning during use of the door entry system.

Additionally, the apartment unit transmit signal must be appreciably attenuated on the output to the TV set to prevent possible reradiation from the flylead and possible direct pickup in the TV IF strip, or blocking of the tuner input stages. An attenuation of about 30dB should be sufficient, which is readily achievable using an untuned LC high pass filter.

For the modulator/transmitter circuit it is preferable to filter the output to the required purity assuming worst case harmonic content.

With regard to uplink and downlink communication frequencies, there is a fairly free choice of transmit frequency for the apartment unit, and so it is possible to use crystals and crystal filters manufactured for the cellular and other mobile telecomms markets.

Stability requirements make the use of crystal controlled transmitters and receivers mandatory to ensure reliable operation.

Choosing a popular frequency, such as 21.400MHz, also makes it possible to use low cost oscillator crystals such as 10.245MHz as the frequency reference for the transmitter modulators, when used with a low drift VCO at 455kHz. The same oscillator, 10.245MHz, can also be used for the receiver chain, reducing the risk of heterodyne whistles caused by low level pickup from the transmitter section, and reducing the overall cost of the design.

Similarly, the door entry unit uplink frequency can be relatively freely chosen, preferably to avoid possible TV picture and sound carriers on the network A convenient frequency, again using low cost 58MHz crystals, is 47.300MHz, which is just above the lower passband limit for the AD408, but comfortably below the picture carrier frequency for the first TV carrier frequency at 48.25MHz (channel E5), which in any case is seldom used.

Selection of the above frequencies is by no means essential for all possible applications of the door entry system. However, the use of these particular values enables the development of designs for the transmitters and receivers without recourse to frequency synthesis.

Modern frequency synthesisers are available which allow the direct programming of a range of transmit and receive frequencies in the factory, enabling the door entry system to work on multiple frequencies, and so allowing more than one transaction or conversation to be carried simultaneously.

With regard to modulation and demodulation of speech and data, the bandwidth of the transmit/receive channels is limited to 3400Hz, narrowband fm modulation being employed for both the uplink and downlink.

The transmit and receive carriers carry both speech and DTMF tones, and also data signals for system control and for call setup/termination. The demodulator outputs must therefore have dual outputs, suitably filtered to minimise the possibility of crosstalk. The use of a short data signalling protocol, together with a low overall data rate and additional protection measures such as repeat signalling, protects againt noise and data corruption affecting system operation.

As both data and speech are carried on common carriers, fully comprehensive muting and mode control functions are provided in both transmitters and receivers; these prevent bursts of data being heard on the speech channels during the setup and the termination of a call.

Typically, the complete exchange of data involved in setting up a call takes place within a few tens of milliseconds of lifting the handset, so the call setup appears transparent to the user.

The door entry unit coax interface unit (which is duplicated in the concierge unit) incorporates the abovementioned carrier detect circuit 52 which checks that the system is not in use before starting the call setup sequence.

Claims (21)

Claims

1. A communication system for a building, the system comprising wiring for feeding television signals to a plurality of occupants of the building, and a telephone system comprising at least one caller station and a plurality of user stations, whereby a caller at the caller station can establish telephone communication with an occupant at a selected user station, said wiring also serving to transmit the telephone signals between the caller station and the selected user station.

2. A system according to claim 1, including a caller station constituted by a door entry station in a door entry system which controls the locking mechanism of a door or gate giving access to the building or grounds surrounding the building.

3. A system according to claim 2, wherein the door entry station includes a door release mechanism which is operated to release the door on receipt of a door release signal from a selected user station.

4. A system according to any of claims 1 to 3, wherein the user stations are located in respective apartments of the building.

5. A system according to any of claims 1 to 4, including a caller station for use by a concierge in the building.

6. A system according to any of claims 1 to 5, wherein the wiring is coaxial cable.

7. A system according to claim 6, wherein the coaxial cable is part of a MATV or SMATV system.

8. A system according to claim 7, wherein the or each caller station is connected into said wiring by means of a diplexer.

9. A system according to any of claims 1 to 8, wherein a caller station and selected user station set up a two-way speech channel by frequency modulation of a carrier.

10. A system according to claim 9, wherein transmission of speech from the caller station to the selected user station is at a first carrier frequency and the transmission of speech from the selected user station to the caller station is at a second carrier frequency.

11. A system according to any of claims 1 to 10, wherein the or each caller station comprises addressing means for addressing a selected user station and for transmitting an address signal to all user stations, each user station having address recognition means for recognising the particular address assigned to that user station.

12. A system according to claim 11, wherein, on recognition of an address signal, a selected user station transmits to the caller station a reply signal, representative of the address of the selected user station, the caller station having means to compare or match the address signal and the reply signal and to establish a speech channel between the caller station and the selected user station when a match is achieved.

13. A system according to claim 12, wherein the speech channel thereby established is secure in that neither can the channel be interrupted, nor can the speech be overheard, by another user.

14. A system according to any of claims 1 to 13, wherein the addressing means is a keypad.

15. A system according to any of claims 1 to 14, wherein each user station has a splitter which splits the TV signals transmitted on the wiring from the telephone signals which are also transmitted on the wiring.

16. A system according to any of claims 1 to 15, wherein each user station has a socket for a standard plug-in telephone, one key of which is allocated to generate a door release signal.

17. A communication system for a building, the system comprising wiring for feeding television signals to a plurality of occupants in the building and a data transfer system comprising a plurality of user stations connected to at least one caller station by said wiring, each user station having respective recognition means operable to open a channel for transfer of data from the caller station to the user station in response to an address signal which contains an address assigned to that user station, the arrangement being such that the caller station selects which user station is to receive the data to be sent by sending to all user stations an address signal which operates the recognition means of the selected user station.

18. A system according to claim 17, which includes calling means at each user station for transmitting a distinctive call signal to the caller station which includes identification means for identifying the user station from which the call signal was sent.

19. A system according to claim 17 or claim 18, wherein a call signal is transmitted in response to the triggering of a smoke alarm or an intruder alarm, and the identity of the user station transmitting the signal is displayed on a display connected to the caller station.

20. A system according to claim 19, wherein the alarms provide a continuous monitoring facility so that a call signal is transmitted upon triggering of the alarm, regardless of whether any address signals have been sent from the caller station.

21. A communication system substantially as hereinbefore described with reference to the accompanying drawings.